U.S. patent number 4,380,735 [Application Number 06/211,938] was granted by the patent office on 1983-04-19 for compensating feedback system for multi-sensor magnetometers.
This patent grant is currently assigned to Her Majesty the Queen in right of Canada, as represented by the Minister. Invention is credited to Malcolm E. Bell.
United States Patent |
4,380,735 |
Bell |
April 19, 1983 |
Compensating feedback system for multi-sensor magnetometers
Abstract
A magnetometer having two or more sensor feedback systems
provides improved accuracy by compensating for the effect of the
feedback field in each system on the other systems. Each system has
a sensor for sensing a magnetic field, a feedback coil associated
with the sensor for providing a feedback field at the latter, a
feedback circuit for energizing the feedback coil in response to
sensing of the magnetic field by the sensor and thereby producing
at the sensor a feedback field for cancelling the sensed field at
the sensor, a differential amplifier for deriving from the feedback
circuit a first electrical signal proportional to the feedback
field, resistors of predetermined resistances selected for
modifying the first electrical signal to electrical signals which
are each proportional to the feedback field at the sensor of a
respective other one of the sensor feedback systems, and circuitry
for combining the first electrical signal from each of the systems
with one of the modified signals of each of the other of the
systems to provide, from each of the systems, a respective output
signal corresponding to the magnetic field sensed by that
system.
Inventors: |
Bell; Malcolm E. (Medicine Hat,
CA) |
Assignee: |
Her Majesty the Queen in right of
Canada, as represented by the Minister (Ottawa,
CA)
|
Family
ID: |
4116205 |
Appl.
No.: |
06/211,938 |
Filed: |
December 1, 1980 |
Foreign Application Priority Data
Current U.S.
Class: |
324/244;
324/225 |
Current CPC
Class: |
G01R
33/025 (20130101) |
Current International
Class: |
G01R
33/025 (20060101); G01R 033/025 () |
Field of
Search: |
;324/225,243-248,253-255,258,260,346 ;33/355R,356,357 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Strecker; Gerard R.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A magnetometer having first and second sensor feedback systems
each comprising: sensor means for sensing a magnetic field;
feedback coil means associated with said sensor means for providing
a feedback field at the latter;
feedback circuit means for energizing said feedback coil means in
response to sensing of the magnetic field by said sensor means and
thereby producing at said sensor means a feedback field for
cancelling the sensed field at said sensor means;
means for deriving from said feedback circuit means a first
electrical signal proportional to the feedback field at said sensor
means; and
means for converting said first electrical signal to a second
electrical signal proportional to said feedback field of said
feedback coil at the sensor means of the other one of said sensor
feedback systems; and
means for combining the first electrical signal of each of said
systems with the second electrical signal of the other of said
systems to provide two output signals corresponding, respectively,
to the sensed magnetic fields at said sensor means.
2. A magnetometer as claimed in claim 1, wherein said first
converting means comprise a resistor having a predetermined
resistance selected to effect the conversion of said first
electrical signal to said second electrical signal.
3. A multi-sensor magnetometer having a plurality of sensor
feedback systems each comprising:
sensor means for sensing a magnetic field;
feedback coil means associated with said sensor means for providing
a feedback field at the latter;
feedback circuit means for energizing said feedback coil means in
response to sensing of the magnetic field by said sensor means and
thereby producing at said sensor means a feedback field for
cancelling the sensed field at said sensor means;
means for deriving from said feedback circuit means a first
electrical signal proportional to the feedback field at said sensor
means; and
means for converting said first electrical signals to modified
signals each proportional to said feedback field of said feedback
coil at the sensor means of a respective other one of said sensor
feedback systems; and
means for combining the first electrical signal of each of said
systems with one of the modified electrical signals of each of the
other of said systems to provide, from each of said systems, a
respective output signal corresponding to the magnetic field sensed
by said system.
4. A magnetometer as claimed in claim 3, wherein said first
converting means comprise a resistor having a predetermined
resistance selected to effect the conversion of said first
electrical signal to said second electrical signal.
Description
The present invention relates to magnetometers and, more
particularly, is useful in gradiometers and multi-sensor
magnetometers.
It is well known that the output voltage of an open loop
magnetometer, i.e. a magnetometer having no feedback coil
associated with a sensor coil thereof, is inherently non-linear and
unstable with respect to a magnetic field being sensed and
measured.
It has previously been proposed to reduce the non-linearity and
instability of the magnetometer output voltage by providing a
closed loop feedback system, in which a sensor coil is associated
with a feedback coil and functions as a null detector.
However, in gradiometers and multi-sensor magnetometers, the use of
a feedback coil associated with each sensor coil has the
disadvantage that each feedback coil will provide a feedback
magnetic field which will affect not only its own sensor coil but
also the other sensor coils, and thus introduce a measurement
error.
It is accordingly an object of the present invention to provide a
novel and improved magnetometer having means for compensating for
such feedback interference.
According to the present invention, there is provided a
magnetometer having first and second sensor feedback systems each
comprising sensor means for sensing a magnetic field; feedback coil
means associated with said sensor means for providing a feedback
field at the latter; feedback circuit means for energizing the
feedback coil means in response to sensing of the magnetic field by
the sensor means and thereby producing at the sensor means a
feedback field for cancelling the sensed field at the sensor means;
means for deriving from the feedback circuit means a first
electrical signal proportional to the feedback field at the sensor
means; and means for converting the first electrical signal to a
second electrical signal corresponding to the feedback field of the
feedback coil at the sensor means of the other one of the sensor
feedback systems; and means for combining the first electrical
signal of each of the systems with the second electrical signal of
the other of the systems to provide two output signals
proportional, respectively, to the sensed magnetic fields at the
sensor means.
The invention will be more readily understood from the following
description of prior art and of a preferred embodiments of the
invention, given by way of example, with reference to the
accompanying drawings, in which:
FIG. 1 shows a circuit diagram of a prior art open loop
magnetometer;
FIG. 2 shows a circuit diagram of a prior art closed loop
magnetometer;
FIG. 3 shows a diagram illustrating the magnetic fields prevailing
at a pair of sensors of a multi-sensor magnetometer;
FIG. 4 shows a sensor coil array;
FIG. 5 shows a circuit diagram of a multi-sensor magnetometer
embodying the present invention;
FIG. 6 shows a sensor coil array; and
FIG. 7 shows a modification of the circuit diagram of FIG. 5.
The prior art open loop magnetometer illustrated in FIG. 1 has a
magnetic field sensor coil 10 connected between ground and a
detector 11. An amplifier 12 is connected between the output of the
detector 11 and, through a voltmeter 14, to ground.
In operation of this magnetometer, a magnetic field which is to be
measured is sensed by the sensor coil 10, and the current induced
in the sensor coil 10 is detected by the detector 11 and amplified
by the amplifier 12. The voltage at the output of the amplifier 12
is measured by the voltmeter 14 as a measure of the magnetic field
sensed by the sensor coil 10.
As indicated hereinabove, such an open loop magnetometer provides a
magnetic field measurement which is inherently non-linear and
unstable with respect to the magnetic field being sensed by the
sensor coil 10 and measured.
It has therefore previously been proposed to counteract such
non-linearity and instability by use of a magnetometer, such as
that illustrated in FIG. 2, employing a closed loop feedback
system.
In this case, the sensor coil, the detector and the amplifier,
which are indicated in FIG. 2 by the same reference numerals as
employed in FIG. 1, are connected in series with a resistor 15 and
a feedback coil 16, which is associated with the sensor coil 10 and
grounded, as illustrated.
The voltmeter 14, as shown in FIG. 2, is employed for measuring the
voltage across the resistor 15.
In operation of the prior arrangement, the output of the amplifier
12 is applied, through the resistor 15, to the coil 16, which
consequently provides a feedback magnetic field. The sensor coil 10
now functions as a null detector, the magnetic field of the
feedback coil cancelling the magnetic field sensed by the sensor
coil 10.
The feedback current fed through the feedback coil 16 is
proportional to the magnetic field nulled by the feedback coil 16
and is linear and considerably more stable than the output voltage
of the open loop magnetometer illustrated in FIG. 1.
However, difficulties arise with the closed loop feedback system
illustrated in FIG. 2 when it is embodied in a gradiometer or
multi-sensor magnetometer employing two or more sensor coils.
Thus, referring to FIG. 3, when two sensor coils 10A and 10B are
employed, the fields acting on these two coils 10A and 10B will be
as represented by the arrows in FIG. 3, in which:
Hz represents an ambient magnetic field;
.DELTA.Hz.sub.A represents an anomaly field at the sensor coil
10A;
.DELTA.Hz.sub.B represents an anomaly field at the sensor coil
10B;
H.sub.FAA represents the feedback field, of a feedback coil
associated with the sensor coil 10A, at the sensor coil 10A;
H.sub.FBA represents the feedback field at the sensor coil 10A of
the feedback coil associated with the sensor coil 10B;
H.sub.FAB represents the feedback field at the sensor coil 10B of
the feedback coil associated with the sensor 10A; and
H.sub.FBB represents the feedback field at the sensor coil B of the
feedback coil associated with the sensor coil 10B.
As will be readily apparent to those skilled in the art, null
detection by the sensor coils 10A and 10B will occur when:
This may be represented as:
The feedback current in the sensor coil 10A is proportional to the
ambient field if H.sub.FBA =H.sub.FAB. However, this only holds
true when .DELTA.Hz.sub.A =.DELTA.Hz.sub.B =0.
If an anomaly exists in the ambient field, i.e. Hz.sub.A is not
equal to Hz.sub.B is not equal to zero, then the feedback current
in the sensor coil 10A will no longer be proportional to the
ambient field at the sensor coil 10A.
The same, of course, holds true for the sensor coil 10B.
Furthermore, it will be readily apparent that the closer the two
sensor coils 10A and 10B are, the larger the measurement error will
be.
FIG. 4 illustrates three sensor coils 100A, 100B and 100C sensing
an anomaly field from ferrous material FM.
The sensor coil 100A is spaced by a distance DO from the ferrous
material FM, the sensor coil 100B is spaced by a distance D1 from
the sensor coil 100A and the sensor coil 100C is spaced by a
distance D2 from the sensor coil 100B.
In this case, at the sensor coil 100A, the ferrous material FM
produces an anomaly field .DELTA.H.sub.A =M/d.sub.o 3.
The corresponding anomaly fields at the sensor coils 100B and 100C
are: ##EQU1## Where M is the magnetic moment of the ferrous
material FM.
In this case:
where H.sub.FAC is the feedback field at the sensor 100C of the
feedback coil associated with the sensor coil 100A, H.sub.FBC is
the feedback field at the sensor coil 100C of the feedback coil
associated with the sensor coil 100B, etc.
Thus, by obtaining voltages proportional to the feedback fields of
each of the feedback coils at each of the sensor coils, the sums of
the ambient and anomaly fields can be determined.
This will be more readily apparent from a consideration of FIG. 5,
in which there is shown a magnetometer having three closed loop
feedback systems indicated generally by Sa, Sb and Sc,
respectively.
Feedback system Sa incorporates the sensor coil 100A, a detector
111A and an amplifier 112A.
The output of the amplifier 112A is applied through a resistor 115A
to a feedback coil 116A, associated with the sensor coil 100A, and
a differential amplifier 120A is connected across the resistor
115A.
The systems Sb and Sc are similar to the system Sa and therefore
the parts thereof have been indicated by the same reference
numerals followed by the suffixes B and C, as appropriate.
The output of the differential amplifier 120A is connected to three
resistors R.sub.AA, R.sub.AB and R.sub.AC, the output of the
differential amplifier 120B is connected to three resistors
R.sub.BA, R.sub.BB and R.sub.BC and the output of the differential
amplifier 120.sub.C is connected to three resistors R.sub.CA,
R.sub.CB and R.sub.CC.
The three resistors R.sub.AA, R.sub.BA and R.sub.CA are connected
to one input of a differential amplifier 121A; the three resistors
R.sub.AB, R.sub.BB and R.sub.CB are connected to one input of the
differential amplifier 121B and the three resistors R.sub.AC,
R.sub.BC and R.sub.CC are connected to one input of the
differential amplifier 121C.
The other inputs of the differential amplifiers are grounded
through respective resistors 122A, 122B and 122C.
Also, the differential amplifiers 121A, 121B and 121C have
resistors 123A, 123B and 123C connected across them as shown.
Referring now to the feedback system Sa, it will be apparent that
this system is similar to that disclosed in FIG. 2, except that the
differential amplifier 120A replaces the voltmeter 14 of FIG.
2.
The voltage V.sub.FCC at the output of the differential amplifier
120A will be proportional to the feedback field H.sub.FCC of the
feedback coil 116A.
This voltage is applied, as indicated above, to the resistors
R.sub.AA, R.sub.AB and R.sub.AC, and these three resistors have
values which are predetermined.
More particularly, the resistors R.sub.AA, R.sub.BB and R.sub.CC
each have the same resistance.
The resistor R.sub.BA has a resistance which reduces the output
voltage of the differential amplifier 120B to a value corresponding
to the feedback field of the feedback coil 116B of the feedback
system Sb at the sensor coil 100A of the feedback system Sa.
Likewise, the resistor R.sub.CA has a value which reduces the
output voltage of the differential amplifier 120C to a value
corresponding to the feedback field of the feedback coil 116C of
the feedback system Sc at the sensor coil 100A.
These resistance values can be determined by individually measuring
the field intensity of the feedback coils 116B and 116C at the
sensor coil 100A.
In this way, the output voltage of the differential amplifier 120A
is compensated for these feedback field intensities from feedback
coils 116B and 116C by summing amplifier 121A, and the output
voltage of the latter represents an accurate measurement of the
magnetic field sensed by the sensor coil 100A.
The output voltages of the differential amplifiers 120B and 120C
are correspondingly compensated.
In this way, measurement errors resulting from feedback
interference between the three feedback systems are corrected.
As will be apparent to those skilled in the art, the invention is
not restricted to the use of coils for sensing the magnetic fields,
but any other suitable magnetic field sensors may be employed.
Also, while the embodiments of the present invention shown in FIG.
5 employs electrical voltages as electrical signals representing
the magnetic fields, other forms of circuitry could be utilized to
provide, for example, electrical currents or a combination of
voltages and currents as such signals.
Furthermore, the above described feedback compensation could
alternatively be effected employing digital circuitry, other forms
of analog circuitry, a combination of digital and analog circuitry
or microprocessor logic.
The embodiment of the invention illustrated in FIG. 5 is suitable
for use in situations in which the feedback field from one of the
sensor coils is opposing the ambient field at another of the sensor
coils. In this case, the sensitive axes of the sensor coils are
vertical and the sensor coil array is also vertical.
However, if the feedback coil associated with one of the sensor
coils is aiding the ambient field at another of the sensor coils,
as would be the case if the sensitive axes of the sensor coils were
vertical and the sensor array were horizontal, as illustrated in
FIG. 6, then the circuitry shown in FIG. 5 may be modified as shown
in FIG. 7.
As can be seen in FIG. 7, there is in this case inserted, between
each operational amplifier 120A, 120B and 120C and the pairs of
resistors R.sub.AB, R.sub.AC ; R.sub.BA, R.sub.BC ; and R.sub.CA,
R.sub.CB, respectively, a respective inverting operational
amplifier 125A, 125B or 125C.
Referring again to FIG. 5, it was mentioned hereinbefore that the
resistances of resistors R.sub.BA and R.sub.CA can be determined by
individually measuring the field intensity of the feedback coils
116B and 116C at the sensor coil 100A.
More particularly, this determination can be effected by the
following sequence of steps:
Firstly, the sensor coils are mechanically aligned so that their
magnetic axes are parallel, and all of the sensor coils are placed
in a zero gradient magnetic field of known value. This will be
referred to hereinafter as the reference magnetic field.
Feedback systems Sb and Sc are disconnected so that no current
flows through either of these coils, and the value of resistor 115A
is adjusted to obtain a voltage of 0 V at the output of amplifier
112A. Feedback system Sb is then reconnected and feedback system Sa
is disconnected so that no current flows through systems Sa and Sc.
The value of resistor 115B is adjusted to obtain a voltage of 0 V
at the output of amplifier 112B.
Feedback system Sc is then reconnected and feedback system Sb is
disconnected so that no current flows through coils Sa or Sb.
Resistor 115C is adjusted to obtain a voltage of 0 V at the output
of amplifier 112C.
As mentioned above, R.sub.AA =R.sub.BB =R.sub.CC and selection of
these resistors can be made using standard operational amplifier
design theory, a value between 1 k.OMEGA. and 100 k.OMEGA., in most
cases, being suitable.
R.sub.BA and R.sub.BC are disconnected from amplifier 120B,
R.sub.CA and R.sub.CB from 120C, and R.sub.AB and R.sub.AC from
120A, and the input side of these resistors is connected to
ground.
Feedback system Sa is connected and feedback systems Sb and Sc are
disconnected so that no current flows through feedback system Sb or
Sc.
R123A is selected so that the maximum magnetic field value to be
measured does not saturate operational amplifier 121A.
The voltage output of operational amplifier 121A for the applied
reference magnetic field at sensor system 100A is measured and
recorded and is referred to hereinafter as the reference output
voltage.
Feedback coil Sb is then connected and feedback system Sa is
disconnected so that no current flows through feedback coil Sa or
Sc.
The reference magnetic field is applied to sensor coil 100B and the
value of 123B is adjusted to obtain the reference output voltage at
the output of amplifier 121B.
In a corresponding manner, the reference output voltage is then
obtained at the output of amplifier 121C by adjustment of resistor
123C and resistors R.sub.BA, R.sub.CA, R.sub.AB, R.sub.CB, R.sub.AC
and no current flows through systems Sa or Sb.
R.sub.AC are likewise correspondingly adjusted to obtain the
reference voltage at the outputs of amplifiers 121A, 121B and 121C
in an analogous manner. Specifically, connect feedback systems Sa
and Sb, disconnect feedback system Sc, and connect the input side
of Rba to the output of 120B. Adjust the value of Rba to obtain the
reference output voltage at the 121A output. Connect Sc, connect
the input side of Rca to the output of 120C, and adjust Rca to
obtain the reference output voltage at the output of 121A.
Disconnect system Sc, connect the input side of Rab to the output
of 120A. Adjust Rab to obtain the reference output voltage at the
output of 121B. Connect system Sc, connect the input side of Rcb to
the output of 120C. Adjust Rcb to obtain the reference output
voltage at the output of 121B. Disconnect system Sb, connect the
input side of Rac to the output of 120A, and adjust Rac to obtain
the reference output voltage at the output of 121C. Connect system
Sb, connect the input side of Rbc to the output of 120B, and adjust
Rbc to obtain the reference output voltage at the output of
121C.
The following resistance relationships then apply: ##EQU2##
* * * * *